Condensed Consolidated Statements of Operations (Unaudited) - USD ($) $ in Thousands	3 Months Ended		9 Months Ended
	Sep. 30, 2019	Sep. 30, 2018	Sep. 30, 2019	Sep. 30, 2018
Income Statement [Abstract]
Net service revenues	$ 1,552,132	$ 1,466,226	$ 4,537,047	$ 4,318,613
Direct costs of services, consisting of payroll, payroll taxes, benefit costs and reimbursable expenses	905,686	855,758	2,645,472	2,528,661
Gross margin	646,446	610,468	1,891,575	1,789,952
Selling, general and administrative expenses	483,555	459,330	1,423,053	1,354,936
Amortization of intangible assets	339	429	1,022	1,334
Interest income, net	(1,230)	(1,196)	(3,768)	(2,937)
Income before income taxes	163,782	151,905	471,268	436,619
Provision for income taxes	46,601	36,663	129,677	115,895
Net income	$ 117,181	$ 115,242	$ 341,591	$ 320,724
Net income per share:
Basic (in usd per share)	$ 1.02	$ 0.96	$ 2.94	$ 2.65
Diluted (in usd per share)	$ 1.01	$ 0.95	$ 2.92	$ 2.62
Shares:
Basic (in shares)	115,181,000	120,115,000	116,203,000	121,112,000
Diluted (in shares)	115,868,000	121,443,000	116,934,000	122,193,000
Cash dividends declared per share (in usd per share)	$ 0.31	$ 0.28	$ 0.93	$ 0.84